Electron beam lithography systems have been used for semiconductor processing for many years. Their application has been somewhat limited however. While electron beam systems can have unsurpassed resolution, their speed of exposing patterns on wafers has historically been inferior to that of other lithography tools, such as optical lithography steppers and scanners. This results in low throughput of exposed product.
Recently, electron beam projection lithography exposure systems have been developed which are expected to offer greatly enhanced throughput. These systems direct a relatively large area electron beam onto a portion (“subfield”) of a reticle containing the pattern associated with a specific process step for a semiconductor device. The electron beam transmitted through the reticle is projected onto a wafer where it forms a demagnified image of the illuminated part of the reticle. The reticle and wafer are mounted on precision high speed stages. By a combination of stage movement and electromagnetic deflection of the electron beam, the entire pattern of the reticle is sequentially transferred to the wafer where it exposes an electron sensitive resist. After exposure, the resist on the wafer is developed, and regions of the resist exposed by the electrons are removed, for a positive type resist; or regions of the resist not exposed by the electrons are removed, for a negative type resist. The remaining resist forms a stencil mask, with which the mask features can be transferred into the wafer by etching or deposition processes.
Throughput is enhanced with these electron beam projection lithography exposure systems because all features of the illuminated part of the reticle are transferred to the wafer simultaneously. Past electron beam lithography systems typically transferred the device pattern to the wafer sequentially, with each exposure step transferring at most a few simple geometrical shapes. Throughput in electron beam lithography systems can also be limited by Coulomb interactions between the electrons in the beam. The electric charge carried by the electrons results in repulsive forces which can defocus and blur the image at the wafer. This puts an upper limit on the maximum beam current the electron beam lithography system can effectively operate at, and this in turn limits the useful throughput of the lithography system. However, several features of electron beam projection lithography systems serve to mitigate this situation, allowing higher beam currents, and correspondingly higher throughputs. For example, by increasing the size of the beam illuminating the reticle, for a given beam current, the current density in the beam is reduced. This reduces the effect of the Coulomb interactions. Also, increasing the numerical aperture, i.e. increasing the electron beam half angle, will also reduce Coulomb interaction effects. However, there is an upper limit to the magnitude of the numerical aperture, because geometrical aberrations, which can blur the image at the wafer, increase rapidly with the numerical aperture.
Currently, an additional requirement of electron beam projection lithography systems is that the intensity of the electron beam be uniform, so that all features are exposed in the resist under the same conditions. If the beam is non-uniform, the dimensions of the features in the developed resist may not be faithfully rendered, and some small features may not even be reproduced. To avoid this situation electron beam uniformities of about 1% or better are required at the reticle.
Furthermore, the total electron charge density (the “dose”) delivered to an illuminated region of the wafer must be controlled rigorously, so that the electron sensitive resist is properly exposed. If the dose is not carefully controlled, the dimensions of the features in the developed resist may not be faithfully rendered, and some small features may not even be reproduced. The dose is normally controlled by carefully timing the exposure of the illuminated region of the wafer. Therefore, the electron beam current from the gun must be constant in time.
The desired large subfield and numerical aperture of electron beam projection lithography systems mean that the electron beam emittance is much greater than in conventional electron beam lithography systems, where emittance is defined as the product of beam size and angular half-width of the angular distribution of the electrons. In the absence of apertures, which can remove part of the beam, or electron optical subsystems which change the beam energy, emittance is a fundamental property of the electron optical system. Thus, electron beam projection lithography systems can require an electron source, or gun, which also possesses high emittance.
One way of obtaining high-emittance from the electron gun is to use a large heated cathode that emits electrons over a relatively large area. For example, if the subfield at the wafer in an electron beam projection lithography system is 0.25 mm, and the numerical aperture is 8 mrad, the emittance as defined above is 2 mm-mrad. This is several orders of magnitude larger than the emittance of conventional electron beam lithography systems. At the cathode of the electron gun, the half angle of the electrons accelerated out of the gun is given approximately by the ratio of the electron's transverse momentum to its axial momentum. The transverse momentum is related to the thermal energy the electron acquires from the heated cathode, while the axial momentum is related to the beam energy imparted to the electron by the gun. If the accelerating voltage of the gun is V and the cathode temperature is T, then the electron beam half angle is equal to approximately (kT/eV)1/2, where k is Boltzmann's constant and e is the electron charge. For an accelerating voltage of 100 kV and a cathode temperature of 2000° K, the beam half angle is roughly 1 mrad. Thus, the emittance requirement calls for a minimum cathode size of approximately 2 mm. However, in order to meet the uniformity requirement, a very uniform electric field to extract the electrons is needed at the cathode, so to avoid perturbing edge field effects the cathode may need to be several times larger in reality. Also a planar cathode is desirable.
Electron emission from the cathode can be limited by the cathode temperature (“temperature limited operation”) or by the accumulation of an electron cloud in front of the cathode, which partially shields the cathode from the accelerating electric field which extracts the electrons into the beam (“space charge limited operation”). Since the electron cloud distorts the electric field, uniform extraction of beam over the emitting part of the cathode will be difficult. Thus, temperature limited operation may be preferred. This puts some constraints on the cathode properties.
The current density j of electrons emitted from a heated cathode is given to a good approximation by the Richardson-Dushman equation,j=AT2exp[−eφ/kT],  (Eq. 1)where A is a constant, and φ is the work function of the material. The work function typically varies with the material's crystalline properties, such as the crystalline orientation at the cathode surface. Thus, the beam uniformity basically requires that the cathode be a single crystal, so that φ is constant. In addition, the temperature of the cathode must be very uniform. For example, for a cathode work function of φ=4 V, a beam uniformity of approximately 1% requires the cathode temperature to be controlled to within approximately 1° K at a cathode temperature of approximately 2000° K.
One method for heating the cathode includes using a heated filament located near the cathode. The filament serves as a source of electron bombardment to heat the cathode to incandescence. Another method includes heating an intermediate cathode with a filament, which then heats the real cathode primarily by radiation.
These methods have provided some success, but they also suffer from certain disadvantages. For example, with a filament, it is difficult to control the area and uniformity of the heating of the cathode. This makes it difficult to obtain temperature uniformity of the cathode.
Additionally, any variations in the local properties of the cathode, such as a variation in the work function of the cathode, can influence electron emission. Also, variations in the thermal emissivity across the cathode surface may create variations in cathode temperature, which will affect the local electron current density. With a filament, there is no way to locally adjust the temperature at different regions of the cathode to compensate for the variation in the work function or emissivity. The heating filament also increases the size and complexity of the electron gun, and the heat generated by the filament adds substantially to the heating within the electron gun, which must be controlled typically with a fluid cooling.
Moreover, a typical filament has a finite lifetime, and must be replaced periodically. During filament replacement, the gun chamber must be opened and the internal components of the electron gun are exposed to atmospheric pressure. Further, the filament heating current and its voltage bias relative to the cathode is controlled by a power supply, which must be biased relative to ground at the cathode voltage. Because the exposure apparatus can operate at high voltages, this biasing has significant impact on the power supply design. At the same time, the power supply to the filament must be stable and noise free, to avoid perturbing the electron beam emitted from the cathode.
Additionally, precision heating of the cathode can be difficult when relatively small incremental amounts of heat need to be added to maintain and/or adjust the temperature of the cathode.
In light of the above, there is a need for a heating assembly that provides substantially uniform heating of the cathode. Further, there is a need for a heating assembly that provides precise heating of the cathode or other object. Still further, there is a need for a heating assembly that minimizes the cooling requirements within the electron gun chamber. Additionally, there is a need for a gun assembly that provides improved electron beam uniformity and stability. Still, there is a need for a gun assembly that is relatively compact, simple, and cost effective to manufacture, assemble and use.